add a howto document about corrupted blob recovery

Extracted from a post by Linus on the mailing list.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/howto/recover-corrupted-blob-object.txt

@@ -0,0 +1,134 @@
+Date: Fri, 9 Nov 2007 08:28:38 -0800 (PST)
+From: Linus Torvalds <torvalds@linux-foundation.org>
+Subject: corrupt object on git-gc
+Abstract: Some tricks to reconstruct blob objects in order to fix
+ a corrupted repository.
+
+On Fri, 9 Nov 2007, Yossi Leybovich wrote:
+>
+> Did not help still the repository look for this object?
+> Any one know how can I track this object and understand which file is it
+
+So exactly *because* the SHA1 hash is cryptographically secure, the hash
+itself doesn't actually tell you anything, in order to fix a corrupt
+object you basically have to find the "original source" for it.
+
+The easiest way to do that is almost always to have backups, and find the
+same object somewhere else. Backups really are a good idea, and git makes
+it pretty easy (if nothing else, just clone the repository somewhere else,
+and make sure that you do *not* use a hard-linked clone, and preferably
+not the same disk/machine).
+
+But since you don't seem to have backups right now, the good news is that
+especially with a single blob being corrupt, these things *are* somewhat
+debuggable.
+
+First off, move the corrupt object away, and *save* it. The most common
+cause of corruption so far has been memory corruption, but even so, there
+are people who would be interested in seeing the corruption - but it's
+basically impossible to judge the corruption until we can also see the
+original object, so right now the corrupt object is useless, but it's very
+interesting for the future, in the hope that you can re-create a
+non-corrupt version.
+
+So:
+
+> ib]$ mv .git/objects/4b/9458b3786228369c63936db65827de3cc06200 ../
+
+This is the right thing to do, although it's usually best to save it under
+it's full SHA1 name (you just dropped the "4b" from the result ;).
+
+Let's see what that tells us:
+
+> ib]$ git-fsck --full
+> broken link from    tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
+>              to    blob 4b9458b3786228369c63936db65827de3cc06200
+> missing blob 4b9458b3786228369c63936db65827de3cc06200
+
+Ok, I removed the "dangling commit" messages, because they are just
+messages about the fact that you probably have rebased etc, so they're not
+at all interesting. But what remains is still very useful. In particular,
+we now know which tree points to it!
+
+Now you can do
+
+	git ls-tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
+
+which will show something like
+
+	100644 blob 8d14531846b95bfa3564b58ccfb7913a034323b8    .gitignore
+	100644 blob ebf9bf84da0aab5ed944264a5db2a65fe3a3e883    .mailmap
+	100644 blob ca442d313d86dc67e0a2e5d584b465bd382cbf5c    COPYING
+	100644 blob ee909f2cc49e54f0799a4739d24c4cb9151ae453    CREDITS
+	040000 tree 0f5f709c17ad89e72bdbbef6ea221c69807009f6    Documentation
+	100644 blob 1570d248ad9237e4fa6e4d079336b9da62d9ba32    Kbuild
+	100644 blob 1c7c229a092665b11cd46a25dbd40feeb31661d9    MAINTAINERS
+	...
+
+and you should now have a line that looks like
+
+	10064 blob 4b9458b3786228369c63936db65827de3cc06200	my-magic-file
+
+in the output. This already tells you a *lot* it tells you what file the
+corrupt blob came from!
+
+Now, it doesn't tell you quite enough, though: it doesn't tell what
+*version* of the file didn't get correctly written! You might be really
+lucky, and it may be the version that you already have checked out in your
+working tree, in which case fixing this problem is really simple, just do
+
+	git hash-object -w my-magic-file
+
+again, and if it outputs the missing SHA1 (4b945..) you're now all done!
+
+But that's the really lucky case, so let's assume that it was some older
+version that was broken. How do you tell which version it was?
+
+The easiest way to do it is to do
+
+	git log --raw --all --full-history -- subdirectory/my-magic-file
+
+and that will show you the whole log for that file (please realize that
+the tree you had may not be the top-level tree, so you need to figure out
+which subdirectory it was in on your own), and because you're asking for
+raw output, you'll now get something like
+
+	commit abc
+	Author:
+	Date:
+	  ..
+	:100644 100644 4b9458b... newsha... M  somedirectory/my-magic-file
+
+
+	commit xyz
+	Author:
+	Date:
+
+	  ..
+	:100644 100644 oldsha... 4b9458b... M	somedirectory/my-magic-file
+
+and this actually tells you what the *previous* and *subsequent* versions
+of that file were! So now you can look at those ("oldsha" and "newsha"
+respectively), and hopefully you have done commits often, and can
+re-create the missing my-magic-file version by looking at those older and
+newer versions!
+
+If you can do that, you can now recreate the missing object with
+
+	git hash-object -w <recreated-file>
+
+and your repository is good again!
+
+(Btw, you could have ignored the fsck, and started with doing a
+
+	git log --raw --all
+
+and just looked for the sha of the missing object (4b9458b..) in that
+whole thing. It's up to you - git does *have* a lot of information, it is
+just missing one particular blob version.
+
+Trying to recreate trees and especially commits is *much* harder. So you
+were lucky that it's a blob. It's quite possible that you can recreate the
+thing.
+
+			Linus